The goal of this project is to field test a technology that would improve the amount of gas that can be extracted efficiently and economically from salt cavern storage.

Performer

RESPEC Inc.

Location:
Rapid City, SD 57703

Background

Product movement to and from compressed natural gas (CNG) storage caverns in salt is accomplished by compression and expansion of the stored gas. The constraint that typically limits the minimum gas pressure for a CNG storage cavern is the potential for salt dilation that can lead to spalling in the cavern roof and/or walls. The gas storage capacity of many caverns can be increased if the minimum level of internal pressure in storage caverns can be reduced without jeopardizing cavern stability. This project investigated the minimum allowable operating pressure of compressed natural gas storage, in salt formations caverns, using a continuum damage mechanics approach.

Geomechanical studies were used to refine and assess the viability of using a state-of-the-art salt mechanics model to predict CNG cavern instability and collapse. The Multimechanism Deformation Coupled Fracture (MDCF) model, originally developed for the disposal of nuclear waste in salt formations, has the capability to predict the onset of dilation. MDCF can also quantify the severity of microfracturing or damage within the salt as a function of time.

Impact

This project was the first to determine the minimum allowable gas pressure of actual CNG storage caverns. The differences for minimum gas pressure determined using both stress-based and damage-based criteria were assessed, with lower minimum gas pressures expected for the damage-based criterion. The ability to reduce minimum gas pressure in CNG salt caverns, using a salt damage criterion, is based on the concept that an accurate prediction of salt behavior can be obtained by an advanced model. This model can track the history of damage and healing of the salt.

Accomplishments (most recent listed first)

A total of 50 mechanical and mineralogical laboratory tests were performed on salt core specimens from two natural gas storage caverns (one existing and one planned), owned by Bay Gas Storage Company, Ltd in the McIntosh salt dome located near Mobile, Alabama. The laboratory tests were used to evaluate the parameters of the MDCF model and to aid in model refinement efforts.

Isometric view of existing and planned Bay Gas caverns

Model refinement was performed to improve the predictive capabilities under operating conditions typical of those experienced by natural gas storage caverns in salt formations. Model enhancements included:

Modification of the healing term to include damage (microfracture) orientation. As a result of this modification, when conditions for damage healing occur, (crack closure and sintering) microfractures recover, as dictated by the orientation of the damage. This update alleviated known deficiencies that impact the healing rate and stresses within the salt surrounding the storage caverns during damage recovery.

Update of the transient creep recovery term to predict, more accurately, the creep rate of salt when the deviatoric stress has decreased (i.e., the gas pressure in the cavern is significantly greater than the minimum gas pressure). Although much of the closure that occurs for a CNG storage cavern occurs when the cavern is at or near minimum pressure, the creep response of hardened salt when the gas pressure is greater than the minimum pressure was found to have a significant impact on the annual closure rate of the caverns.

Reformulation of the dilation boundary term to reflect a material that is weaker under triaxial extension than triaxial compression states of stress. The original formulation of the MDCF model did not capture this known behavior of salt and resulted in potentially exaggerated predictions of minimum gas pressure.

Due to time and economic constraints, laboratory testing at stress states other than triaxial compression was limited. Only one, successful complete creep test, in triaxial extension was completed. As a result, complete development of the model was not fully realized during this project. However, development of the model was sufficient for use in numerical calculations of the storage caverns. Using the proposed method compared to a conventional stress-based method to determine minimum working gas pressure, results showed that the working gas capacity of the existing cavern could be increased by 18 percent, and by 8 percent in the planned cavern.

Comparison of Predicted Closure Histories of Bay Gas Well No. 2 Using the Original and New Recovery Formulations.